There are numerous applications in which it is necessary or desirable to control some form of interlock or other electrical safety circuit in accordance with the speed of a rotating shaft. In many instances, these are vehicular applications; for example, an electrical interlock to assure closing the doors of a passenger vehicle whenever the vehicle is moving above a predetermined threshold speed, or an electrically actuated interlock mechanism to preclude shifting a vehicle transmission into reverse gear whenever the vehicle is moving in a forward direction at even a limited threshold speed. Similar speed switch control requirements, based upon the rotational speed of a shaft or like rotary member, are also commonly encountered in connection with machine tools and other industrial equipment.
The requirements imposed upon speed switch controls of this general type, particularly those employed in vehicular applications, are frequently quite severe. Thus, the control may be subjected to high levels of vibration and to substantial shock forces. Electrical transients of substantial magnitude may be encountered. Because the control does not perform a primary operational function, it is frequently subject to severe cost limitations. In addition, it is highly desirable that the electrical connections to the control be as simple as possible, preferably constituting a simple two-terminal connection, to minimize cost and to facilitate replacement when necessary.
A variety of different speed switch controls have been devised for use in applications of this kind; many of these controls are based upon an input signal derived from a small AC generator driven by the shaft or like rotary member to be monitored. In many instances, the circuits of these devices have been undesirably complex, particularly because operation is based upon the frequency of the AC input signal, with a consequent need for a frequency/voltage conversion stage as a part of the control circuitry. In general, known speed switch controls have been undesirably high in cost and difficult to construct in a form rugged enough for vehicular applications, in large part due to the circuit complexities introduced by frequency/voltage conversion. In addition, many of the known speed switch controls require three or four or even more terminal connections in order to provide for an external power supply, apart from the load, and have presented substantial problems with respect to providing adequate transient protection. Again, these problems are particularly acute in speed controls applied to vehicles.